Web MVC, REST, Validation & Errors

Overview

How an HTTP request becomes a Java method call, how Spring reads/writes JSON, how @Valid rejects bad input, and how @ControllerAdvice centralises error responses.

Official: Spring MVC.

Table of contents

1. Request lifecycle
2. @RestController vs @Controller
3. Mapping URLs to methods
4. JSON in and out
5. Validation with @Valid
6. Global exception handling
7. ProblemDetail (Spring 6)
8. Interview questions
9. Summary

1. Request lifecycle

1. Embedded Tomcat receives the HTTP request.
2. Spring’s DispatcherServlet finds the matching controller method by path + verb.
3. Spring binds path variables, query params, headers, body into method parameters.
4. The method runs and returns a value.
5. For @RestController, Spring serialises the return value to JSON (default via Jackson) and sets Content-Type: application/json.

2. @RestController vs @Controller

@RestController is literally @Controller + @ResponseBody on every method.

3. Mapping URLs to methods

@RestController
@RequestMapping("/api/orders")
public class OrderController {

    @GetMapping("/{id}")
    public Order get(@PathVariable Long id) { ... }

    @GetMapping
    public List<Order> list(@RequestParam(required = false) String status) { ... }

    @PostMapping
    public ResponseEntity<Order> create(@Valid @RequestBody CreateOrderRequest body) {
        Order saved = service.create(body);
        return ResponseEntity
            .created(URI.create("/api/orders/" + saved.getId()))
            .body(saved);
    }
}

@GetMapping / @PostMapping are shortcuts for @RequestMapping(method = ...).

4. JSON in and out

• Incoming JSON → Java: @RequestBody on the parameter; Jackson deserialises.
• Outgoing Java → JSON: return any POJO/record; Jackson serialises.

Use ResponseEntity when you need to control HTTP status or headers (e.g. 201 Created with a Location header).

5. Validation with @Valid

1. Put Bean Validation annotations on the DTO:

public record CreateOrderRequest(
    @NotBlank String customerId,
    @Positive int quantity
) {}

2. Add @Valid to the controller parameter:

@PostMapping
public Order create(@Valid @RequestBody CreateOrderRequest req) { ... }

3. If invalid, Spring throws MethodArgumentNotValidException before your method runs.

@Validated (Spring’s version) adds validation groups — only needed for advanced cases.

6. Global exception handling

Instead of try/catch in every controller, declare a @RestControllerAdvice:

@RestControllerAdvice
public class ApiExceptionHandler {

    @ExceptionHandler(NotFoundException.class)
    public ResponseEntity<ProblemDetail> notFound(NotFoundException e) {
        var pd = ProblemDetail.forStatusAndDetail(HttpStatus.NOT_FOUND, e.getMessage());
        return ResponseEntity.status(HttpStatus.NOT_FOUND).body(pd);
    }
}

One handler, consistent error shape across the whole API.

7. ProblemDetail (Spring 6)

ProblemDetail is Spring’s built-in implementation of RFC 7807 (application/problem+json):

Stable shape → clients and gateways handle errors uniformly.

8. Interview questions

Q: @RestController vs @Controller? A: @RestController adds @ResponseBody so return values are written to the response body (usually JSON). @Controller is for server-rendered HTML.

Q: What does @Valid do? A: Runs Bean Validation on the object; invalid input throws MethodArgumentNotValidException before the handler runs.

Q: What is @RestControllerAdvice? A: A global bean holding @ExceptionHandler methods that apply to every controller.

Q: How do I return 201 Created with a Location header? A: ResponseEntity.created(URI).body(...).

Q: What is ProblemDetail? A: Spring 6’s Java type for RFC 7807 problem+json error responses.

9. Summary

Next steps

• HTTP status codes — 4xx/5xx semantics
• Data & transactions

Happy Learning

Next: Data Access (JPA) & Transactions